In optical spectral analysis, the composition, the condition or other properties of the object to be examined are examined by means of the interaction of electromagnetic radiation with the surface and at the volume of said object. In this context, the wavelength-dependent reflection, transmission, absorption and scattering properties of materials are exploited.
Various variants of arrangements for performing spectroanalytical examinations have been known.
A first variant illuminates the sample with the entire polychrome spectrum of a light source, for example a halogen lamp or a thermal radiator. The light that has started to interact with the sample is then broken up into its spectral constituents within a monochromator, and detected by means of a radiation detector or photodetector. To this end, what may be used is either a mechanically movable dispersive element (grating, prism) or an arrangement of several radiation detectors. What is disadvantageous in the first possibility is utilization of a movable element, which results in increased overhead for the overall system. On the other hand, use of an arrangement of several detectors entails a relatively large amount of effort and is relatively expensive, particularly in the infrared wavelength range. In addition, due to the high optical power, the sample is heated and, thus, the measurement is influenced. A further disadvantage is the limited miniaturizability of the overall system. It is restricted, among other things, by the sizes of the radiation sources.
In a second variant, the light of a light source is broken up into its spectral constituents before starting to interact with the sample. For this purpose, a monochromator may be used as well, which, in turn, contains mechanically movable parts. Following the interaction with the sample, the light is detected using an individual detector. The sample is exposed to a comparatively small radiation intensity. The architecture is comparatively costly and mechanically delicate. In addition, in this case, too, the radiation source restricts the miniaturization of the overall system on the basis of system integration.
In a third variant, the electromagnetic radiation is generated within a very small spectral range only. For this method, one predominately uses such lasers whose wavelengths may be changed by tunable resonators are predominately used. What is advantageous is the high intensity within a very small wavelength interval. What is disadvantageous is the limitation to specific wavelength ranges with corresponding laser activity, increased effort devoted to the system due to the high levels of mounting accuracy, and the high price resulting therefrom. In addition, mechanically movable elements, such as gratings, are sometimes used, which entails sensitivity toward mechanical environmental influences. Also, when irradiating rough surfaces, the high spatial coherence leads to interference effects, so-called speckles, which may result in measurement errors in the detection.
In a fourth variant for a wide-range near-infrared spectral analysis, an electron cyclotron (storage ring) may be used, for example, which due to its size cannot be configured to be portable.